This invention relates in general to fastener driving devices and more. More specifically, to control circuits for electrically operated fastener driving devices such as for example electric tackers.
An example of an electrically operated tacker is shown in the commonly assigned U.S. patent application Ser. No. 294,422, of Bernecki et al, filed Aug. 19, 1981, now U.S. Pat. No. 4,417,681. That application is hereby incorporated by reference into the present specification. For tackers, such as the Bernecki et al tacker it is appropriate to energize the tacker's solenoid for no more than a single cycle of commercially available A.C. power. Various analogtype circuits are known and used for this purpose, such as, for example, U.S. Pat. No. 3,215,864--Doyle et al (Nov. 2, 1965), the teachings thereof being incorporated herein by reference. In the Doyle et al circuit, a unidirectional controlled conduction device, such as a silicon controlled rectifier (SCR) is used to close a power circuit so that current will flow in an electrical load such as the tacker's solenoid winding. Current flow through the solenoid winding causes a mechanical power stroke to be produced for effecting a tacking operation.
Known control circuits generate a gate signal for triggering the SCR on the first properly poled half cycle of an applied A.C. voltage after the randomly timed actuation of a switch by a user. The control circuit also includes means for preventing the application of a further gate signal to the SCR until the switch is released and then reactuated by the user.
These known actuating circuits use some sort of analog scheme for generating the gate signal for triggering the SCR to "fire" the solenoid. For example, in the Doyle U.S. Pat. No. 3,215,864, a charge is built up on a capacitor. When the switch is actuated this charge is used to generate the gate signal. This is a rather typical approach to generating the firing signal.